This invention relates generally to methods and apparatus for well logging and more specifically relates to methods and apparatus for injecting a radioactive tracer element into the fluid flow within a well to assist in the determination of parameters concerning the movement of such fluid flow.
Wells drilled for use in the production of oil or gas may be utilized either as producing wells, from which the oil or gas is obtained, or as injection wells, through which fluids, such a polymer compounds, are forced into the earth formations surrounding the well to force the oil or gas through the formations to thereby stimulate production in nearby wells. Wells may often be drilled as producing wells and subsequently converted to use as injection wells when such change in usage becomes economically preferable.
In a producing well, the oil or gas may enter the well from one or more depth horizons or zones within the earth formations surrounding the well. When the oil or gas is being produced from more than one zone it is desirable to determine how much of the fluid is being produced from each zone. This determination may be made by measuring the velocity of the well fluid at various depth locations within the well, preferably at locations between the known producing zones. From such fluid velocity measurements and knowledge of both the diameter of the well at each depth location and of the diameter of the logging instrument used to measure the fluid velocity, the volumetric flow rate of the well fluid at each depth location may be determined, thereby facilitating a determination of the proportional contribution of each producing zone to the total flow rate of the well.
An analogous but reverse situation is presented in the case of injection wells. When polymers, or other fluids are being injected, or pumped, into the well, it is desirable to determine the proportion of the injection fluid which is entering each zone within the well so as to determine if the desired zones are receiving the injection fluid. By measuring the velocity of the injection fluid and by determining the fluid flow rate therefrom at various depth locations within the well, the proportion of the injection fluid entering each zone may be determined.
One means which has been utilized by the oil and gas industry to make such fluid velocity measurements, and in turn such flow rate determinations, has been to inject a tracer element, such as a radioactive isotope, into a well fluid flow and to measure the time required for the tracer element to traverse a known distance within the well. In the case of a radioactive tracer this may be accomplished by injecting the tracer element from a logging instrument upstream in the fluid flow from a pair of suitable radiation detectors, such as Geiger counter tubes or scintillation counters, spaced a known fixed distance apart along the longitudinal axis of the logging instrument. The time between the passing by the tracer element of the first detector to the passing of the second detector may then be utilized to determine the velocity of the well fluid. Apparatus commonly used in the oil and gas industry for injecting tracer elements into the well fluid flow have typically ejected the tracer element in a generally lateral direction relative to the longitudinal axis of the well bore, such ejection typically being accomplished by forcing the tracer element through one or more jets or nozzles located on the periphery of a logging instrument. This method of tracer ejection has been found to be unsuited for use in production wells with relatively low flow rates, rates less than 300 barrels per day and/or in injection wells using relatively high viscosity fluids, such as polymers. In these types of wells the tracer element ejected from an instrument tends to be propelled across the dimension of the well and to become stuck to the side of the well. Additionally, the low fluid flow rates tends to allow the tracer material to be dispersed within the fluid flow, or to form an elongated bubble of tracer material, making accurate detection difficult.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a method and apparatus whereby tracer elements may be accurately and repeatedly placed in a desired proportional location across the fluid flow column within a well and by which characteristics of the movement of such fluid may be determined.